Growth of Atlantic cod (Gadus morhua L.) with different haemoglobin subtypes when kept near their temperature preferenda

The effect of temperature on growth and feed utilization was studied over 56 days in Atlantic cod of different haemoglobin subtypes [designated Hb-I(1/1), Hb-I(1/2), and Hb-I(2/2)]. The fish, held in triplicate tanks, were reared at 6 and 14 °C (mean initial body weight, 218 and 212 g, respectively). These temperatures are close to the preferred temperatures of Hb-I(2/2) (8.2 °C) and Hb-I(1/1) (15.4 °C). Temperature had a significant effect on SGR, with cod of all haemoglobin variants growing best at 14 °C (P < 0.05). There was no significant effect of Hb-I⁎ subtype on SGR. Since cod of type Hb-I(2/2) had a higher SGR at 14 °C than at 6 °C this suggests that the temperature preferendum may not provide a good indicator of optimum growth temperature. Significant differences between the Hb-I⁎ variants were detected in several nutritional indicators. At 14 °C, the feed efficiency was poorer in the Hb-I(1/2) cod (0.99 ± 0.08) than in the Hb-I(1/1) and Hb-I(2/2) fish (1.35 ± 0.05 and 1.52 ± 0.07, respectively). Energy retention was significantly lower in the Hb-I(1/2) fish (19.5 ± 3.7%) than in the Hb-I(2/2) fish (43.8 ± 1.8%) at 14 °C. Regardless of the Hb-I⁎ subtype, retention of macronutrients and conversion efficiencies were better in fish reared at 6 than 14 °C.